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Industrial Power System Design

Source: digikey
Category: Power Mana...
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文章创建人 Lisa Eitel

Original Title:Industrial Power System Design

  Power systems consist of electrical and electronic components to control electrical power delivered to automated machinery. These systems and their subcomponents ensure that machines maintain safe, precise, and efficient operation — no matter the fluctuating conditions of power feeds or the machine itself.

  All industrial automation (battery powered or otherwise) traces back to its source, the alternating current (ac) of the electric-utility transmission and distribution grid. The generators used in these utility-supply operations can just as easily be designed to produce a direct current (dc) — and in fact, a wide array of devices in industrial automation run off dc power. However, ac is used because it’s easily stepped up and down (through the use of transformers) to various voltages. This is vitally important, as very high voltages are needed for maximally efficient electrical-power transmission and distribution over long distances.


Figure 1: Electrical utilities use ac power for transmission, as it’s easily stepped up and down by transformers. However, power-quality equipment is necessary to compensate for the various unpredictable inconsistencies of utility power — to protect and optimize the performance of industrial-automation machinery that runs off this electrical power.

  Consider some of the ways in which utility power into automation machinery can be compromised.

  Blackouts: A power outage or blackout is a complete loss of electrical power from some electrical-supply fault at one or more utility generation, transmission, distribution and substation locations. Critical infrastructure depending on electrical power (including water treatment plants, hospitals, data centers, and sensitive industrial-automation operations) typically have on premises some sort of generator-based backup power to enable continued operation during grid power outages.

  Brownouts: This undervoltage condition lasts for a longer period of minutes or hours. This may be caused intentionally if it’s necessary to reduce power consumption in an emergency. In a brownout, resistance devices such as heaters and incandescent lights simply reduce their output in proportion to the reduction in voltage. This isn’t harmful to these devices. Some motors will also behave in a similar way while others may draw additional current leading to overheating and potentially burnout. Power supplies and digital devices can be affected in different ways and may malfunction.

  Frequency instability: Most non-renewable (and some renewable) electricity generation today is produced using turbines to rotate an electromagnetic coil (the rotor) within a series of heavy copper bars making up the stator. Each time the rotor’s magnetic field passes one of the stator bars, it induces electrical current. As the rotor’s positive and negative poles pass the stator, the electric current alternates direction. A generator rotating at 3,000 rpm produces ac with a frequency of 50 Hz or (in the U.S.) 60 Hz. The three copper-bar sets in standard generators produce the three phases seen in the electrical distribution system.

  The frequency of every generator across an entire national grid must be synchronized to maintain a stable grid frequency — typically to within 1% or better. The challenges involved in this are often overlooked. Fluctuating electrical demand on the grid causes differences between the generator’s electromagnetic load and the input mechanical power. This causes rotor rpm to vary and the grid frequency to fluctuate.

  The inertia of large heavy generators dampens the rate of frequency changes. Thermal power generation (both based on biomass and fossil fuels) provides this inertia and good frequency response by rapidly changing the power delivered to the generator. However, rising adoption of wind and solar power is greatly increasing the challenge of maintaining utility frequency stability. Although it’s possible to switch generators on and off to give some degree of frequency response, the proliferation of small operators complicates coordination of this at the grid level. There is also little inertia in wind and solar energy sources, so expensive ancillary frequency-stability services are increasingly required. These services employ battery, molten salt, flywheel, and superconducting loop modes of energy storage.

  Surges: Voltage within the distribution system is typically held to within 5% or better. It’s controlled at the grid level by producing and absorbing reactive power. The term voltage surge is an ambiguous term that refers to several different overvoltage conditions. The power quality standard IEEE 1100-2005 provides more clearly defined terms — transients and swells.

  Transient spikes last from microseconds to a few milliseconds. An impulsive transient is a sharp rise in over-voltage which can be caused by a lightning strike or a motor being turned off. An oscillatory transient involves the voltage alternately swelling and then shrinking very rapidly. Transients may also refer to undervoltage conditions over the same time periods. They can be caused by inductive or capacitive loads being turned on or off.

  In contrast, a swell is an overvoltage that lasts longer than a transient, typically for a few cycles, with a voltage 5 to 10% above normal. Voltage sag or dip is an undervoltage condition which lasts for the same time period, sag is therefore the opposite of swell.

  Power-supply forms and functions

  Addressing many of the above utility-power issues are advanced power supplies. As explained earlier in this article, these devices supply electric power to an electrical load. The primary focus of this article section is standalone power-supply products for automation. Refer to the digikey.com page on board-mount power supplies for more information on PCB and other supplies for integration into electronic components such as computers and controllers.


Figure 2: Shown here are a range of power-quality systems and power-supply components to serve automated facilities — from the power mains to individual automation axes. SolaHD STV100K surge protectors (A) are rated for various voltages and short-circuit currents. K-factor harmonic transformers (B) feature conductors that keep the system within its rated temperature range even when carrying the harmonic currents of nonlinear loads. S5K uninterruptible power supplies or UPSs (C) maintain steady power even during utility blackouts. Offline UPSs (D) supply power in such situations to less critical applications such as facility offices. Some surge-protective devices (E) can be installed at the utility-service entrance to protect the whole facility. LVGP transformers (F) come in versions for outdoor or indoor installation. MCR constant-voltage power conditioners (G) and (L) impart further functions to ensure consistent power into machinery subsections. Solatron Plus line conditioners (H) work at the facility point-of-use level to deliver automatic over and under voltage regulation of three-phase local facility supplies. SDN-C ac-to-dc converters and SDU UPSs (I) are DIN-rail-mounted components that work together to provide industrial automation, process, and material-handling applications with right-sized power and power-boost capabilities on the plant floor. SCP 100S24X-CP power supplies (J) are ac-to-dc converters that are rugged enough for field mounting (directly on machinery) which is useful for distributed control schemes. SDN2.5-24-100 power supplies and surge protectors often pair with SBE industrial control transformers (K) to ensure top reliability of automation elements involving precision motion control. SHP dc power supplies (L) paired with MCR constant-voltage power conditioners excel on standalone machine stations. (Image source: SolaHD)

  Standalone power supplies (sometimes called electric-power converters) convert source electric current to a current, voltage, and frequency that’s usable by the attached electrical or electromechanical load — such as a motor, for example. Power supplies also serve to limit load-drawn current to within safe limits — and cutting off input power upon detection of an electrical fault. Yet another function is that of power conditioning for the protection of attached electric loads through the mitigation of electromagnetic interference (EMI) and voltage-surge effects.

  Still other power-supply functions (such as those related to energy storage and power-factor correction) depend on the electric load.

  Just consider the wide array of today’s devices requiring dc to operate. These necessitate power supplies capable of ac-to-dc conversion. A rectifier performs this conversion. While ac is constantly changing direction, dc flows in a single direction. That’s why rectifiers are often based on diodes, which work like one-way valves — only allowing current to pass in a single direction. Power supplies may also incorporate energy storing and smoothing components such as capacitors and inductors to filter their output and ultimately produce more constant voltage.


Figure 3: Transformers on ac power circuits use two coils in close proximity to step voltage up or down. Input current passes through one coil, and that induces an output current in the other coil. Differing numbers of windings in the two coils cause the output voltage to differ from the input voltage. The caveat is that such a transformer only works if the current is constantly changing — as with alternating current. (Image source: Design World)

  As detailed above, most basic power supplies consist of transformers (to change the voltage) and rectifiers if a dc output is required. However, modern power supplies perform many other functions. These include voltage regulation, surge protection, and filtering noise and harmonics. For example, the primary function of uninterruptible power supplies (UPSs) is to provide continuous power during blackouts, but UPSs also serve many of the aforementioned functions to improve power quality. Such power supplies include several other electric and electronic subcomponents to execute these higher-level tasks.

  Just consider the difference between linear power supplies versus switching power supplies for ac-to-dc power conversion — and the importance of their voltage regulation function. There are two fundamental ways such power supplies execute voltage regulation.

  1. Linear power supplies work by first using a transformer to convert the input ac power to the required voltage. A rectifier then performs the basic ac-to-dc conversion and additional filtering produces a more level dc. This output must be somewhat higher than the actual output voltage to prevent undervoltage conditions.


Figure 4: The output of a simple rectifier consisting of only diodes isn’t smooth. The output current doesn’t change direction as in alternating current, but it still exhibits considerable fluctuations. (Image source: Design World)

  Voltage regulation is via the dissipation of excess power as heat. A regulating device serves as a variable resistor to continuously adjust a voltage divider so that the output terminals are held at a fixed voltage — and excess voltage is placed over the energy-dissipating resistor. When the regulating device is in parallel with the load, it’s known as a shunt regulator. When the regulating device is in series with the load, it’s known as a series regulator. Continuously dissipating energy as heat means that linear power supplies have low efficiencies of 60% or so.

  2. Switching power supplies or switched-mode power supplies (SMPSs) use more sophisticated electronics to achieve 80% or higher efficiencies. Instead of continuously dissipating energy, they rapidly switch power between the load and energy-storage onboard components such as inductors and capacitors. Before this type of voltage regulation can be performed, the electrical power must first be converted to ac of very high frequency — typically above 20 kHz. If the input is ac, it’s first rectified before being inverted to produce the high-frequency ac.

  Next, the power supply executes voltage conversion — and because of the high frequency, a much smaller transformer can be used than would be required for a comparable linear power supply. This means that an additional advantage of SMPS over linear supplies is that they are much smaller and lighter. Refer to the Digi-Key tutorial covering Qualtek-brand switch-mode power supplies for more on this.

  3. Though beyond the scope of this article, note that there is a third hybrid voltage-regulating option for power supplies. These are particularly useful in supplying power to smaller motor-driven axes of industrial-automation installations.

  Common regulatory requirements

  Several standards specify requirements for power supply systems. For example, by the end of 2020, all computer and audio-visual equipment will need to comply with IEC 62368-1: A/V information and communication technology equipment Part 1: Safety requirements. This places additional requirements on the power supplies built into such equipment as well as external power supplies for such equipment. It’s purely concerned with safety and does not make any requirements for the performance or function of equipment. Although there seems to be some leeway in the U.S. for equipment that has already been shown to meet previous standards, the changes will be more strictly enforced in Europe as EN 62368-1.

  Another standard defining hazard-based safety engineering (HBSE) methodology involves identifying potentially hazardous energy sources, classifying them by level of danger, identifying safeguards, and then qualifying safeguards. The danger of energy sources is classified into three levels:

  • Class 1: Exposure to the electrical-energy source is not painful but may be detectable — Ignition not likely.

  • Class 2: Exposure to the electrical-energy source is painful but will not cause injury — Ignition is possible but limited in growth and spread of fire.

  • Class 3: Exposure to the electrical-energy source will result in injury — Ignition is likely as is rapid growth and spread of fire.

  Another set of regulations affecting power supplies is that of the National Fire Protection Association (NFPA). These include the NFPA 110 Standard for Emergency and Standby Power Systems, which defines many requirements for the safe installation of power supplies. Of primary concern is ensuring that backup power systems can reliably support vital safeguarding systems (such as emergency lighting or pump systems) in buildings.


  Facilities engaged in industrial automation necessitate electrical and electronic power-quality components in an array of locations — from the utility-service entrance of the plant all the way to the furthest reaches of the most remote machinery’s components. Only then can automated processes remain safe and reliable — no matter the current utility-line conditions.

  The choice of power supply (especially at the field level of industrial-automation installations) depends on application load requirements — as well as its need for power quality and protection from outages. For most applications, switched-mode power supplies have replaced linear supplies; although when it comes to specifying a UPS, the choice is often not so straightforward.


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